2 BULLETIN 1038, U. S. DEPARTMENT OF AGRICULTURE. 
total suppression of chlorophyll, but since a yellowing or bleaching 
of normally green parts may result from a wide variety of causes 
chlorosis in itself is a symptom rather than a disease. There are. 
for example, the chloroses of etiolation and of the normal autumnal 
ripening of leaves. Moreover, a yellowing of chlorophyll frequently 
follows upon some types of insect injury, such as that of root aphids 
on the peach and the grape, and it is a constant accompaniment of 
certain diseases caused by parasitic bacteria and fungi, such as bac- 
terial black-rot of cabbage and Fusarium wilts of cabbage and potato. 
Again there are the more or less general chloroses due to unfavorable 
soil or climatic conditions, and finally those infectious chloroses of 
obscure origin which present a fairly regular sequence of pathologi- 
cal signs, including fundamental derangements in both metabolism 
and morphogenesis. 
Some of these chloroses are true diseases in the restricted sense. 
Others are not diseases except under a broad application of the 
term, and certain forms of chlorophyll restriction are clearly not 
diseases at all. 
There are, however, two fairly well marked types of chlorophyll 
disturbance which are usually included under the chlorotic group of 
plant diseases. These are (1) the infectious chloroses which are 
communicable through expressed plant juices or through those juices 
as directly transmitted within the living plant tissues, and (2) the 
noninfectious chloroses due to unfavorable soil or atmospheric con- 
ditions. 
The present study of pecan rosette deals with the histology and 
cytology together with the sequence of gross symptoms of the dis- 
ease. In order to place the results of this study in proper relation 
to other diseases of this type it is necessary to review briefly some of 
the work of other investigators. 
CHLOROSES DUE TO SOIL OR ATMOSPHERIC CONDITIONS. 
With respect to those chlorophyll changes due to physical or 
chemical conditions of soil or atmosphere it is difficult to say just at 
what point the normal state ends and chlorosis begins. 
Certain plants prefer an acid condition of the soil, others tolerate 
it. others are restricted for their optimum development to neutral 
or alkaline situations. Nevertheless, neither macroscopic nor micro- 
scopic examination of such a plant as field sorrel {Rumex acetosella 
Linn.), for example, would give a clue to its acicl-soil toleration. In 
response to certain environmental changes, however, Transeau (78) 
has shown that this species does develop anatomical changes. In 
moist situations, with soil and air temperature approximately iden- 
tical, the leaves of this species are relatively large, with a loose 
